IN OUR SKIES: Life starts out as, eventually returns to, dust

It is now widely accepted among scientists that the universe began in an enormous explosive-type event that is generally referred to as the "Big Bang." According to data that has been accumulated from various investigations, including space missions such as the Wilkinson Microwave Anisotropy Probe that was launched in 2001, the time of the Big Bang has been determined as having occurred roughly 13.7 billion years ago.

Immediately after the Big Bang, the universe was extremely hot and dense. Once the environment had cooled to the point where matter could exist, that first matter was simply in the form of subatomic particles like protons and electrons. A proton can also be considered as the nucleus of a hydrogen atom, and while the temperatures during the first few minutes after the Big Bang were much too hot for the formation of actual atoms, the conditions were such that the fusion of protons into heavier elements could take place.

About 20 minutes after the Big Bang, the universe had cooled to the point where nuclear fusion could no longer occur. But by that time about 25 percent of the universe's mass was in the form of helium as a result of this fusion process. Small amounts of a few heavier elements, primarily lithium and beryllium, had also been formed. After a little less than 400,000 years the universe had cooled further to where atoms could begin to form from the existing atomic nuclei and the surrounding electrons.

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Much of this matter was in the form of a dense cloud of hot gas. Over time -- a few hundred million years -- small instabilities in this gas cloud began to contract under the force of gravity, and thus the first stars began to form. These first stars were composed almost entirely of hydrogen and helium, although as nuclear fusion in their cores commenced heavier elements began to form -- particularly with more massive stars where helium could fuse into carbon. In even more massive stars, carbon could then fuse into still heavier elements.

For the most massive stars, heavier elements began to form via the nuclear fusion process, with each step occurring more rapidly than the last. Eventually, after approximately 10 million years, iron is formed. If the star is massive enough it will then try to fuse iron into yet heavier elements.

Due to the atomic structure of iron, however, this process requires energy rather than supplies it, and the star's core receives this energy from collapse of the star itself. This process happens extremely rapidly, and things soon reach a point where it suddenly can collapse no further. In consequence the entire star blows itself apart in an explosion known as a supernova.

It is during those seconds immediately after a supernova explosion that all natural elements heavier than iron are formed. These elements are expelled away from the star, enriching nearby clouds. Meanwhile, the blast waves from the explosion can trigger the formation of newer generations of stars containing the elements produced during the earlier stars' lifetimes. It was an event of this nature occurring roughly five billion years ago that caused our sun -- a third-generation star -- to begin forming.

As the sun formed it was also surrounded by a disk containing gas and dust particles that had been enriched by earlier generations of stars. Over time these particles collided and stuck together, eventually producing objects a few miles across called "planetesimals" -- what we today think of as asteroids and comets.

During the first few tens of millions of years after the sun's formation, many of these planetesimals collided and stuck together, in the process growing larger, until Earth and the various other planets in our solar system had formed.

All of the substances that constitute our Earth came from these comets and asteroids, including the water and the organic (i.e., carbon-containing) molecules within our bodies. We can, in fact, detect the presence of these substances within large gas and dust clouds in interstellar space, and in the planet-forming disks surrounding stars currently in the act of forming.

In a very real sense, then, the atoms inside our bodies formed billions of years ago in the interiors of very massive stars; we are, literally, "star dust."

After we die and our bodies decay, the materials in our bodies return to the Earth, perhaps eventually becoming contained within future life forms centuries, millennia, maybe even millions of years from now. Over a much longer time frame, approximately five billion years, while our sun is not massive enough to explode as a supernova, its atmosphere will expand to the point where Earth will be consumed and destroyed, and its materials incorporated into that atmosphere.

That expanding atmosphere of the sun will eventually disperse into interstellar space, eventually being incorporated into the existing gas and dust clouds. At some point, perhaps many millions of years thereafter, another supernova will trigger the formation of another star in those clouds. The process begins again, with planets forming, and perhaps eventually the incorporation of the molecules that were once in our bodies into the bodies of some new life forms -- life forms that may themselves look at the stars and wonder about their own origins.

And the cycle continues for as long as the universe exists in anything resembling its present form. For this brief instant in cosmic time, we are here and we are alive.

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His website is www.earthriseinstitute.org.